Jul. 18, 2023
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Tick-borne rickettsial diseases in the United States include Rocky Mountain spotted fever, caused by Rickettsia rickettsii; other spotted fever rickettsioses, including Rickettsia parkeri rickettsiosis, Pacific Coast tick fever, and rickettsial pox; ehrlichiosis, most commonly caused by Ehrlichia chaffeensis; other ehrlichioses, due to Ehrlichia ewingii and E muris eauclairensis; and anaplasmosis, caused by Anaplasma phagocytophilum. African tick bite fever is caused by Rickettsia africae, and it is commonly seen in travelers returning from sub-Saharan Africa.
This article discusses Rocky Mountain spotted fever, anaplasmosis, ehrlichiosis caused by Ehrlichia chaffeensis (hereafter ehrlichiosis), and African tick bite fever. Although the general clinical presentations are similar to nonspecific influenza-like illnesses, there are distinctive differences. Unlike Rocky Mountain spotted fever, anaplasmosis and ehrlichiosis are less likely to present with rash, and both have a unique histopathology or morulae, which are intracytoplasmic inclusions in peripheral white blood cells. African tick bite fever characteristically presents with one or more cutaneous eschars at the site of inoculation.
• Rocky Mountain spotted fever, ehrlichiosis, anaplasmosis, and African tick bite fever are transmitted by ticks.
• Rocky Mountain spotted fever is the most common rickettsial illness in the United States.
• Early symptoms of Rocky Mountain spotted fever are nonspecific and include high fever, headache, and a macular rash.
• Rocky Mountain spotted fever is more likely to cause neurologic illness and death than anaplasmosis, ehrlichiosis, or African tick bite fever.
• Anaplasmosis and ehrlichiosis have symptoms that are similar to Rocky Mountain spotted fever, but they less commonly include rash.
• Anaplasmosis is characterized by morulae in granulocytes.
• Ehrlichiosis is characterized by morulae in monocytes.
• The illness of African tick bite fever is similar to that of Rocky Mountain spotted fever, but patients have one or more distinctive cutaneous eschars.
• Doxycycline at 2.2 mg/kg orally or intravenously BID (maximum 100 mg BID) is the first-line treatment for adults and children of all ages with rickettsial infections, including pregnant women, and should be started while awaiting laboratory confirmation.
Rocky Mountain spotted fever. Rocky Mountain spotted fever was originally described in Montana and Idaho in the 1870s. The extensive investigative work of Dr. Howard Taylor Ricketts between 1906 and 1909 led to the identification of the etiologic agent, Rickettsia rickettsii, and confirmed the tick as the vector (42). Wolbach confirmed that the bacterium responsible for Rocky Mountain spotted fever was carried by wood ticks and that it was an obligate intracellular pathogen (51).
Anaplasmosis. A phagocytophilum was first identified in humans in 1994. It was originally thought to be a new species of Ehrlichia, and it was named Ehrlichia phagocytophilum; the disease it caused was called human granulocytic ehrlichiosis. In 2001, E phagocytophilum was renamed A phagocytophilum, and the disease was renamed human granulocytic anaplasmosis, or more commonly, anaplasmosis (35). Anaplasmosis has been reportable in the United States since 1999.
Ehrlichiosis. The first instance of human infection with E chaffeensis was documented in 1986 in a man bitten by ticks in the state of Arkansas in the United States. He presented with fever and confusion and then developed anemia and thrombocytopenia. Morulae within monocytes were identified, and treatment with doxycycline was curative (48). Ehrlichiosis became a reportable disease in the United States in 1994.
African tick bite fever. Despite its first description in 1911, African tick bite fever was erroneously attributed to Mediterranean spotted fever until 1992, when the disease and the causative agent, R africae, were described; they became official in 1996 (27). African tick bite fever is endemic in rural sub-Saharan Africa, the Caribbean, and Oceana (45). It is responsible for about half of published travel-related rickettsial infections (45; 17).
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